Amplifier



June 20, 1939. H M, OWENDOFF 2,162,744

AMPLIFIER Filed Jan. 7, 193'/l 24 23 25 Il f AMPLIFIER WITH FEEDBACK CONNECT/0N AND CORREC TIVE NETWORK.

REL A Tl VE RESPONSE /N DB MPL/F/ER WITH FEEDBACK CONNECT/0N OPENED T u -5 ND CORRECT/VE NETWORK OPENED T "yf I l l l I l I l l Il l I l l 3 456789/ 2 3 456789/ 2 3 4.56789/ 30 /00 /000 l0, 000

FREQUENCY /N CYCLES PER SECOND /Nl/EN TOR H. M. OWENDOF F AHORA/Ey V` Patented June 20, 1939 UNITED STATES PATENT OFFICE AMPLIFIER Harold M. Owendoff, Brooklyn, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation .of New York f l Application January 7, 1937, Serial No. 119,374

3 Claims.

This invention relates to wave amplifying sysl tems.

An object of the invention is to control transmission properties of such systems, as for example transmission efficiency and distortion.

In one specific aspect, theiinvention is applied to an amplifier for speech. or other audio frequency signal waves, having an output transformer that, because of its economical design,

l0 -has its primary inductance so low as to tend to Y ing of the transformer. .This voltage, and consequently the amount of the negative feedback, decreases with decrease in the primary impedance of the transformer. Thus, in the lower portion of the signal frequency range, compensation is` effected for decrease of amplifier gain with frequency decrease, and'in the upper portion of the range, compensation is effected for such decrease of amplifier gain with frequency increase as is due to primary capacity of the transformer. Then, in accordance with the invention, to compensate for the effect of the secondary capacity and the leakage inductance in lowering the Y v amplifier gain with frequency increase in the upper portion of the range, further controlof the amount of the negative feedback may be provided by suitable design of the feedback circuit. For example, when it is desired to accentuate the decrease of negative feedback with frequencyincrease in the upper portion of the range, the voltclaims.

In the drawing, Fig. 1 is a circuit diagram of an amplifier 4embodying the specific form of the Zinvention referred to above; and l Other objects and aspects of the invention will be apparent from the following description and4 Fig. 2 shows curves facilitating explanation of the operation of the amplifier of Fig. 1.

The amplifier of Fig. 1 comprises two vacuum tubes I rand 2 in cascade connection. vIt amplifies signal wavessupplied to tube I from incoming 5 line or circuit 3 through input transformer 4 and gain control potentiometer 5, and transmits the amplified waves through output transformer 6 to outgoing line or circuit 1. The signals may be,

for example, audio frequency signals such as'y 10 speech or music.

The tubes are coupled by an interstage coupling circuit comprising a coupling resistor 8, stopping condenser 9 and grid leak resistor I9.

Plate voltage for the tub-es is supplied through 15 lter II.

Plate current for tube I passes through resistors I2 and 8, tube I, and resistors I3 and It. Plate current for tube 2 passes through the primary winding I5 of transformer 6, tube `2, and resistor I6. s 20 Resistors I4 and I6, by-passed by condensers Il' and I8, respectively, supply grid biasing potentials for tub-es IV and 2respectively, and resistor I3 also supplies a grid biasing potential for tube I.

The resistor I2, a resistor I9, and by-pass condensers 20, 2I and 22 supply steady potentials for biasing the screen grids of the tubes from the plate voltage supply source, as usual.

A feedback connection 23 including a resistor 24 and a stopping condenser 25 of negligibly low reactance connects the plate of tube 2 to the end of resistor I3 at the cathode of tube I. Thus, since the cathode of tube 2 is connected to the other end of resistor I3 through the by-pass condensers I8 and I'I, and since the lower end of s Winding I5 is connected to the cathode of tube 2 through by-pass. condensers 22, the alternating current voltage across winding I5 is-v fed back across resistor I3 through the feedback connection. This feedback is negative feedback or gainreducing feedback. Negative feedback is advantageous for example for reducing the modulation and noise in the amplifier, as pointed out in the paper by H. S. Black on Stabilized feed-back;

amplifiers, Electrical Engineering, January 1934, 4.-.

pages 114-120, and the voltage amplification for propagation once around the feedback loop may be of a large `order of magnitude than unity, for

\ obtaining large modulation reduction, as pointed out in that paper. 50,

The feedback connection deriving the voltage to be fed back from across the winding I5 renders it feasible to employ an output transformer of highly economical design yet avoid having the low i primary reactance that is entail-ed by the low cost 55;.

' would cause objectionable lowering of the trans-,1

lil

mission efficiency of the amplifier with frequency increase in the upper portion of the band, for instance as indicated by curve A of Fig. 2, wherein each of the curves A, B and Clmay indicate the Variation of the response of the amplifier with frequency under the conditionnoted at the curve.,

The corrective network referred to in Fig. '2 isI condenser 26 and resistance 21 connected.Y in series across the resistance I3. Whenrthe circuit of this corrective networkY is open or the network is omitted from the vramplifier, but the feedback connection is functioning, the yvariation of the amplifier gain with frequency may be,

`for instance, as indicated by curveV B.v The amount of the negative feedback lvaries with frequency in response to the Vchange in the impedance of the winding I5 with frequency or in other words variesI with frequency in accordance Vwith the variation in the transmission,,efficiencyr of the amplifier with frequency, so as to. reduce the latter variation.

For instance, at'lowv frequenciesnwhere, with decreasing frequency, the inductive reactance of the transformer becomes so low that without the feedback connection'the amplifier gain decrease would become too great, theloweringof the ,in-. ductive reactance or the lowering ofthe imped-,l

ance of winding I E decreases the amount of negal.tive feedback and so reduces the gain decrease,

the gain Variation with frequency then being, for example, as indicated in curve B.

Similarly, at high frequencies` where, -withincreasing frequency, theprimary capacitivereactance of the transformer tends to become so low that without the feedback connection the amplifier gain decrease would become too great, the lowering of the capacitive reactanceV or the lowering of the impedance of Win-ding I5 decreases the amount of negative feedback and so reduces the gain decrease, the gain variation with frequency then being, for example, as indicated in curve B.

Curve C indicates a further improvement or, correction at the high frequencies, to corripenrfff` sate for the effect of the secondarycapacity'and the leakage nductance of the transformer in, lowering the amplifier gain .with frequencyincrease in the upper portion of the signal fre quency range, such increase yin improvement bef` ing obtained by connecting in circuit the corrective network consisting of condenser` Zvand" resistor 2. This capacitive network` .decreases the voltage across resistor 2l appreciable in comparison to the voltage across condenser 26, the corrective network decreases the amount of negative feedback with increase of frequency, for frequencies below those at which the reactance of condenser 26 isI negligible compared to the impedance of resistor 21.

At these high frequencies, therefore, the gain of the amplifying portion of the circuit is made relatively great compared with the gain at other portions of the band, and this increased gain overcomes the relatively high loss in the output circuit coupling at high frequencies.

Even in the case in which the tube 2 is a screen grid tube or a pentode tube, supplying the space current of the tube rthrough the winding I5 tends to lower the nductance of the transformer greatly, or greatly increase ,the cost of the transformer fora given value of nductance. It has been found that the feed-back circuit of Fig. l can readily reduce by a factor of ten, for example, the nductance required in the transformer in order to keep the gain variation of the amplifier within given limits. Y

What is claimed is:

1. A wave translating system comprising an audio frequency amplifier, an input circuit for supplying an audio frequency lband of signal waves thereto, said amplifier having an output transformer whose primary nductance is sufciently low to cause objectionable lowering of the transmission efficiency of said system with frequency decrease in the lower portion of said band and whose leakage nductance and secondary capacity are sufficiently large to cause objectionable lowering of the transmission efficiency of said system with increase of frequency in the upper portion of said band, an impedance in said input circuit, anda feedback path from said output transformer to said inputVV circuit, said path comprising a resistor connected4 in series with said impedance across the primary winding of said transformer, said resistor feeding back across said impedance a voltage derived from the primary winding of said transformer in gain-,reducing phase, said Vimpedance comprising a resistance shunted by a capacitive impedance",Vr said inductanceof said transformer tending to cause the negative feedback with increase of frequency in the upper portion ofthe signal` frequency, range, by causing the impedance of the circuit consisting of resistor I3 and the corrective net-V work in parallel to decrease with the frequency increase and thus reduce the ratio of the voltage across this circuit to the Voltage across the resistor 24, with the frequency increase.Y A

When the frequency is high enoughto render..

the shunting effect of the corrective network upon the resistance I3 appreciable andrende'r said gain-reducing -feedback `to decrease with frequency in saidl lower portion of said band, and said capacitive impedance reducing the ratio of the voltage across the firstmentioned impedance tothe voltageacross said resistor ,with frequency increase in ,saidupper portion ofsaid band andthereby decreasing said gain-reducing feedback with frequency increase in said upper portion of said4 band, to compensate for said lowering oftransmission eiciency caused by said leakage inductance and secondarycapacity.

2. A wave translating system comprising an amplifier for a frequency band of signal waves, input and output circuits therefor, an impedance connectedas a two-terminal impedance .in said input circuit, lsaid output circuithavingreactance that tends to lower theamplier gain with frequency increase ina portion `ofisaid band, anda feedback connectionfrom .saidoutput circuit to said input circuit, said connection comprising a resistance connected in series with. said ,imperi-.

ance acrosssaid output circuitV with one end of said resistance at the potential of one of .said

terminalsy for signal frequencies, said resistance yderiving from said output circuit a voltage proportional to that across said output circuit and feeding said derived voltage back throughsaid resistance across said two terminals of said impedance in said input circuit in gain-reducing phase, and said impedance comprising capacitive reactance that reduces the ratio of the voltage across said impedance to the voltage across said resistance with frequency increase in said portion of said band, thereby reducing said gainreducing feedback with said frequency increase and compensating for said tendency of said reactance of said output circuit to lower the amplifier gain.

3. A wave translating system comprising two electric space discharge devices each having an anode, a cathode and a grid, a source of waves to be translated bysaid system for connectio-n to one of said devices, an output transformer for i said system having its primary winding connected between the anode and the cathode of said other device, and circuits coupling said devices in a closed feedback loop having the voltage amplification for propagation once around the loop large compared to unity to reduce distortion introduced by the system below the distortion without feedback, said circuits comprising means connecting the anode of said one device to the grid of the other, an impedance connecting said cathodes, a connection from the grid of said one device through said source to a point on said impedance, and a feedback connection comprising a resistance extending from the terminal of said winding electrically nearest the anode of said other device to a point on saidimpedance electrically closer than said first point to the cathode of said one device, said feedback connection and the portion of said impedance between said second-mentioned point and the cathode of said other device being connected in series across said winding, and said impedance comprising a resistance between said points and, in shunt to the latter resistance, a capacity and a resistance in series.

HAROLD M. OWENDOFF. 

